1. Field of Invention
This present invention relates to a defect inspection and classification of semiconductor wafers having a plurality of dies. And more particularly, the present invention relates to a method for calculating yield impact contributed by specific defect type in order to predict yield loss effectively.
2. Description of Related Art
Defect monitor methodology typically involves defect inspection and classification of defect types through optical microscope and Scanning Electron Microscope (SEM) review in order to control particles and defects that are inexpectant in production. In order to shorten the respond time and implement appropriate improvements for integrated circuits manufacturers, it is fairly important to establish an estimation scheme of various types of defects on the yield issue. This estimation scheme also allows an engineer to prioritize the defect reduction plan in the fabrication process. The first literature for yield estimation was reported by Wallmark in 1960. Since then, extensive work has been done concerning the different defect density distribution and probability function. However, the methodology for computing the effect of defects on yield was not well developed.
Defect counts are usually used to track excursions and set priorities for defect reduction efforts, but the total defect counts information on its own or even including further characterizations for each single defect by optical microscopes or SEM is not sufficient to control excursion. Moreover, it is neither sufficient to assign an accurate number of yield loss according to the existing yield prediction models to each process for complex chip designs. The review of all inspected defects may as well delay the manufacturing process of semiconductor chips and yield learning cycles significantly.
The concept of “kill ratio” is introduced to estimate the effect of defects on yield loss contributed by specific defect type. It can be defined as the upper limit for yield estimation that could be obtained if the defect type was the only factor affecting the yield. An available prior methodology provides a method following the steps:                (a) The step of computing kill ratio for a given process inspection includes dividing hits on failed dies by defect counts for the process inspection may be included.        (b) The step of determining a number of dies to be killed by considering kill ratio for the dies with hits to weight the defects of each die. It may include the steps of determining a number of dies to be killed by considering all dies with hits, initializing the number of dies to be killed to zero, and modifying the number of dies to be killed. This modification can be done by calculating a kill ratio between a kill ratio for each process inspection having hits and a sum of the kill rates for all process inspections having hits and adding the ratio to a previous number of dies to be killed. The previous number of dies to be killed is being obtained at a previously considered die and repeating the step of modifying until all dies have been considered.        (c) The step of the foregoing modification is preferably performed using the formula:       k    i    =            k              i        ,        previous              +          m      ·                        r          i                                      ∑                          i              =              1                        n                    ⁢                      r            i                              where ki is a newly calculated number of dies to be killed for inspection process i, ki,previous is the previously calculated number of dies to be killed for the defects of inspection process i, ri is the kill rate for a given inspection process i, n is the number of processes which provided hits to the dies to be killed and m is the defect counts of a give type for a die being considered for the inspection process i.        (d) The step of determining a yield loss for each inspection process based on the number of dies to be killed and a total number of dies may include calculating the yield loss by dividing the number of dies to be killed of a given inspection process by the total number of dies of the semiconductor wafer. The method may further include the step of determining a killing probability yield loss for each inspection process based on the number of dies to be killed for each inspection process and a total number of defects for the inspection process.        
Unfortunately, practitioners in the semiconductor industry still hazily utilize the kill ratio concept. To obtain useful yield impact information, it is desirable to establish a succinct “kill ratio” definition in order to filter out excursions in a control table, and show an effective expression for yield loss contribution for specific defect type.